1. Field of the Invention
The present invention relates to metal detectors, and more particularly to a compact hand-held metal detector having a three dimensional detection region, and visual, audible, and tactile indication modes.
2. Description of Related Art
Concealed weapons are a concern for law enforcement personnel, emergency medical personnel, and others. Typically, law enforcement personnel must frisk an apprehended suspect to search for weapons, thereby risking punctures and/or cuts from sharp, and potentially contaminated, concealed objects.
Emergency medical personnel may accidentally find concealed weapons during physical examinations resulting in similar risks. Additionally, an unconscious or semi-conscious patient may awaken and attack the medical personnel with undiscovered concealed weapons.
Metal detectors are known in the art, and can be utilized by law enforcement and emergency medical personnel to locate concealed weapons in the above circumstances. In addition, metal detectors are used in industrial and other applications for inspection and screening for metal in and on various objects, and other applications. For example, automobile manufacturers may want to inspect the carpeted interiors of automobiles for metal contaminants prior to shipment. Hunters may want to inspect game for bullets and shell fragments during dressing and prior to consuming the game.
However, due to the size of available metal detectors, they are typically inconvenient to carry and use. The large size of metal detectors is in part due to large transmit and receive coils needed increase sensitivity to detect small objects, and the requirement to keep the coils away from the receiver electronics to avoid interference and false alarms.
In addition, to date, metal detector's target acquisition indications have included audio oscillators that produce loud audible signals. If a suspect observes law enforcement or medical personnel preparing to use a large metal detector that may locate a concealed weapon, or realizes that a metal detector has located a weapon by hearing a loud audible alarm indication, the suspect may panic and pull the weapon endangering the law enforcement or medical personnel. In addition, audible alarms may be difficult to hear in very noisy environments.
Some metal detectors have been equipped with earphones in an attempt to eliminate the loud audible alarm signal. However, earphones are not practical for field use because of the restrictions imposed by the earphone wire possibly becoming tangled or broken.
Most metal detectors known in the art utilize a tuned oscillator incorporating a separate receive and transmit coil. Metal is detected by frequency changes or shifts in the oscillator that occur when the transmit and receive coils are brought near a metal object. However, generally for detection, the detector must be moved in relation to the object. If the detector stops moving, even while in the vicinity of a metal object, the object will no longer be detected.
In addition, most detectors have a single active detection region requiring the detector to be held in a specific orientation to position metal objects in the active detection region. The directional limitation of metal detectors increases the risk that a metal object could be missed by not properly orienting the detector's active detection region in relation to the metal object.
An example of a small metal detector is disclosed in U.S. Pat. No. 4,423,377 to Podhrasky (the '377 patent). The device of the '377 patent includes a transmit coil and a receive coil in planar relationship, and a planar circuit board adjacent and parallel the transmit and receive coils with electronic components strategically positioned thereon. The transmit coil radiates energy at approximately 90 khz. A conductive target produces a change in the magnetic coupling between the transmit and receive coils. When the rate of change of magnetic coupling exceeds a threshold value, a visual and audible signal is produced. However, if the detector remains motionless relative to the conductive object, the rate of change of magnetic coupling drops below the threshold value, and the alarm signal(s) stop.
The positioning of the electronic components on the circuit board reduces magnetic coupling between the transmit and receive coils and the electronic circuit of the device. Because of the configuration of the coils and placement of the circuit board, the scan area is inherently directional. The scan area is essentially perpendicular to a plane containing the coils and circuit board, and points in the direction away from the coils opposite the circuit board. The scan area is disclosed as covering about 80% of the length of the case. Therefore, objects in the active scan area must be directly below the unit's housing.
The audible signal can be turned off, leaving only the visual signal to alert the operator of an acquired conductive target. Because the directional requirements of the active detection region requires the unit to be held in a specific orientation for detection, a visual only alarm could be difficult to monitor while conducting a search. A detection indication could be missed if the visual indicator were held in a position that prevented or restricted viewing.
The device disclosed in the '377 patent clearly exhibits several of the limitations discussed above.
Therefore, there is a need for a metal detector that is sensitive enough to detect small ferrous and non-ferrous metal objects, but is compact to be easily carried and used. The metal detector should include an alarm indication that prevents the suspect from being alerted to the discovery of a concealed weapon, but is easy to use without missing detection indications. In addition, there is a need for the active detection region of the metal detector to be active in all planes to ease operation of the detector.